Silicon nitride has been widely used for various structural parts such as vehicle engine parts, cutting tools, and abrasion resistance materials such as roller bearings, and the like, since it is excellent in hardness, mechanical strength, and thermal resistance, and also chemically stable. Recently, in all of these fields, not only have the levels of performance required of materials become extremely high but also the degrees of processing accuracy required of those materials have become strict. As a result, when these materials are used for products, the costs of the products become higher as the costs of processing are higher, which has been the greatest factor in preventing expansion of the markets.
Thus, various processing methods have been proposed. Among them, the most frequently used method is a technique of electric discharge machining that is performed in a state wherein electric conductivity is afforded to a silicon nitride composite sintered body by dispersing conductive particles into a matrix composed of silicon nitride and grain boundary phase.
For example, in Ceramics 21: pp 719–725 (1986), it is described that electric discharge machining is made possible by dispersing conductive particles of from 20 to 40% by volume into Si3N4 to make a silicon nitride material with electric conductivity. However, the surface roughness of such silicon nitride materials deteriorates after the electric discharge machining, and surface cracks occur which are attributed to thermal shock and electric discharge at the electric discharge machining. Thus, the resultant materials cannot be used for practical application because of their low mechanical properties unless grinding or polishing is carried out to remove their surface cracks after the electric discharge machining.